As more specific operational and performance demands are made on radio communication devices, a larger number of clock dependent components, such as micro controllers are used. In general, a micro-controller executes a pre-determined algorithm using a clock reference signal. The clock reference signal is generated by an oscillator circuit having a crystal oscillator. These oscillator circuits generally produce harmonics of several orders. These harmonics, albeit at low levels, generally interfere with the operation of the communication devices in which they operate. Harmonics having frequencies close to the operating frequencies of receivers fool such receivers to think that they are signals to be received. Under these conditions the receiver unquiets and proceeds to receive the signal. This phenomenon is known as self quieting in the radio communication industry and is very annoying to the user. In general, self quieting occurs when the receiver receives a harmonic of the micro-controller clock frequency and processes that as if it where a valid received signal, thereby quieting itself (hence the name self quieting). Detection circuitries in the form of filters have been designed to stop this phenomenon. The success of these filters is limited for they can only be designed to filter a particular signal and not all harmonics. One method of curtailing this self-quieting problem is implemented by a circuit shown in FIG. 1.
Referring to FIG. 1, an oscillator circuit 100 is shown to include an amplifier 112 and a crystal 110. Also shown are resistor 118, and capacitors 114 and 116 as gain control elements. These are the necessary elements to produce oscillation. A switch 102 comprising an inductor 104 and a transistor 106 is included in the path of the crystal 110 to provide a shift of the output frequency, when desired. In a general sense, the output 120 produces a signal based on the frequency of the operation of the crystal 110. In the event that this signal is on the frequency of the receiver in which this oscillator is operating, the switch 102 under the control of the control line 108, is turned on resulting in the addition of the inductor 104 in the path of the crystal 110. With the induction of the inductor 104 added to the reactance of the circuit 100 and the stiff nature of the quartz crystal 110, the frequency of the output signal 120 shifts by a predetermined value. In general, however, this value is not sufficient to avoid self quieting problems. In low band applications where a more significant shift in terms of parts per million of frequency is desired, this solution comes short of meeting its desired objectives. An improved oscillator circuit is therefore desired having means to shift the frequency of the output signal by a sufficient amount.